What secret sides to human nature do therapists see that non-therapists would be surprised by?

I'll give you my favorite one: People CREATE precisely what they try hardest to avoid.

Germophobes put paper on public toilet seats and let it fall on the floor when they're finished because they don't dare touch it. They don't flush because they refuse to touch the flush handle. So when you walk into a public restroom with toilet paper all over the floor and a toilet full of wretched feces you blame the low-class slobs but it's actually the neat freaks who make the mess!

People who are afraid of unfairness will unintentionally treat people unfairly in order to ensure that no one is taking advantage of them.

It's a fascinating phenomenon which I have seen in hundreds of people for decades.